1. Field of the Invention
The present invention relates to the speed control of electric motors and more particularly concerns switching operations and mechanisms for providing a smoothly varying and efficient speed control.
2. Description of Prior Art
Speed control of electric motors in diverse applications has been the subject of design and development over a period of many years. It is known that the speed of a direct current electric motor increases with an increase of applied voltage, decreases with an increase in resistance in circuit with windings and increases as magnetic flux entering the armature decreases. For battery operated devices, various methods of switching multiple batteries to and from series, parallel and series-parallel arrangements are known and described in a number of U.S. Patents, including U.S. Pat. Nos. 640,968; 969,513; 1,189,947; 3,264,540; 3,566,985 and 3,718,848. However, voltage switching, or more specifically, battery switching, alone cannot provide an adequate range and smoothness of transition among varying speeds.
Several types of field weakening or decrease of magnetic flux have been employed as exemplified by U.S. Pat. Nos. 789,377; 1,189,947; 1,291,233; 2,472,646; 3,264,540; 3,283,227 and 3,348,111. In these arrangements, various field windings of the motor are switched from series connection to parallel connection, taking advantage of the fact that inductive coils in series produce stronger magnetic flux because all of the current passes through each coil. Further, with the coils in parallel only part of the current passes through each coil and accordingly, a weaker magnetic field, a field weakening, is produced to to provide an increased speed. In some of this group of field winding switching patents, different ones of the windings are selectively switched on or off as desired. Again, field weakening as the sole method of motor speed control will not provide a practical and economical speed control with smooth transition and wide range of speed variation. Further, unique variations and combinations of motor windings required for field weakening schemes of the prior art greatly add to motor costs.
U.S. Pat. No. 789,377 shows an arrangement of selectively shunting field coils in a manner to provide three different speeds. However, shunting as distinguished from complete disconnection, is not adequate for many uses, since shunt contacts may vary in resistance particularly as contacts wear, become eroded, and otherwise foul, so that a shunt may actually increase in resistance as it ages. Still further, the various prior art methods of switching from series to parallel or parallel to series connections of different coils require actuation of a minimum of three independent contacts for the one switching step, thus requiring ganged switches or plural solenoids.
A widely employed motor speed control involves the use of a rheostat or continuously variable resistance placed in circuit with the field windings or with the field windings and armature windings. As the resistance in circuit with the field winding is decreased, speed of the motor increases. However, increasing the resistance, as required for lower speeds, decreases the efficiency of power usage since the greater the resistance in the circuit the greater amount of electric power that is wasted by the control mechanism.
Electronic switching systems employing thyratron power controls or silicon control rectifier (SCR) control systems eliminate or minimize many of the above mentioned disadvantages. They provide a wide range of smoothly varying speed and substantially eliminate inefficiencies of resistive speed control. Nevertheless, such systems are complex and costly, particularly where speed control of vehicle motors are concerned. Electronic valves such as the solid state SCR's capable of handling the large current required in vehicle operation, and having a life long enough to satisfy requirements of such vehicle operation, are considerably more expensive than other types of control systems. Further, the more complex electronic control systems require more difficult and costly upkeep and repair.
Combinations of different methods of speed control have been suggested, but only in arrangements that are complex, expensive, inefficient and difficult to repair. The patent to Beach, U.S. Pat. No. 969,513, illustrates use of different types of speed control employing a complex drum type switch that is difficult and costly to manufacture and maintain and entails a complicated sequence of switching steps that will cause inefficiency and lack of reliability.
The patent to Dannettell, U.S. Pat. No. 3,264,540, shows a combination of speed control arrangements requiring eight different double pole switches and a pair of diodes to provide varying speed by varying the resistance in series with the windings, switching the batteries from series to parallel connection and switching the fields from series to parallel connection. The large number of different switches required greatly adds to the cost and complexity of this system while concomitantly decreasing reliability and ease of maintenance and repair. Particularly where solenoids are employed to perform the switching action, major compromises are required between costs and reliability. Reasonably economical solenoids capable of handling powers required for speed control of electric motor driven vehicles have a relatively short lifetime and will generally fail after an unacceptably small number of operations. Since such switches in a speed control system are operated with great frequency, requiring at least one operation of a switch for each speed change in either direction, solenoid operated switches for this purpose must be capable of operating over millions of cycles. Such stringent requirements can be met only with solenoid devices that cost too much to be economically acceptable. On the other hand, the complex multitude of switching operations required of a device such as illustrated in Dannettell does not lend itself to mechanical switching.
In a system of the prior art, presently employed for speed control of golf cart motors, a first speed range is provided by a resistor and a parallel connection of a pair of 18 volts batteries, a second speed range is provided by the parallel 18 volt batteries alone, a third speed range is provided by the resistor with the 18 volt batteries connected in series to provide 36 volts, and a fourth speed range is provided by the 36 volt series connected batteries without the resistor. As previously mentioned, the use of the resistor in circuit with the windings wastefully dissipates power. Further, the higher the voltage with which the resistance is employed, the greater the waste. In the just mentioned prior art system, this inefficiency is recognized and provision is made to automatically shift from the inefficient third speed position (employing a resistor in series with 36 volt supply) directly to the efficient high speed use of the 36 volt supply alone. However, it is found that in certain types of applications, such as motors for golf carts, for example, as much as 40% of the driving time may be at the next to the highest speed. This most frequently used speed is generally in the order of 7 to 8 miles an hour in a golf cart having a top speed of 10 miles an hour. Thus, where resistive control is used, as in the just described prior art system, the most commonly used speed is the speed at which there is the greatest waste of power. Nevertheless, in the prior system, it is found necessary to use this third speed employing 36 volts and resistor speed control to enable a smooth transition between the speeds available at 18 volts and the high speed available at 36 volts.
Accordingly, it is an object of the present invention to provide electric motor speed control that substantially avoids or minimizes the above mentioned disadvantages without significantly compromising cost of manufacture and repair, operating lifetime, and efficiency or smoothness of operation through the various speed ranges.